The aim of this research is to compare preoperative, initial and later postoperative psychological capacities of patients with hemispherectomy for infantile hemiplegia and for brain tumor to provide data on the principles underlying the organization, disorganization and reorganization of cerebral functions in the development of the brain. Systematic studies of effects of hemispherectomy for lesions incurred at different stages in the development of the brain will provide data for assessing 1) The nature, rate, and extent to which the "functional plasticity" of the infant brain diminishes with advancing age; 2) The extent to which each hemisphere participates in certain language and nonlanguage functions and specializes in others as the brain matures; and 3) The limitations and principles underlying reorganization of cerebral functions following removals of either hemisphere for lesions incurred at different stages in the development of the brain. BIBLIOGRAPHIC REFERENCES: Smith, A. Development and Reorganization of Language and Cognitive Functions after Hemispherectomy in Children and Adults. 21st International Congress of Psychology Abstracts. Paris, July 18-25, 1976. Burklund, C.W. and Smith, A. Language and the cerebral hemispheres: Observations of verbal and non-verbal responses during eighteen months following left ("Dominant") hemispherectomy. Neurology, 1977 (in press).